TECHNICAL FIELD
This invention relates to certain polyacetal compositions containing at least one acetylated hindered amine light stabilizer, said compositions being characterized as having good stability upon exposure to light. Hereinafter, the term "hindered amine light stabilizer" shall be referred to as "HALS."
Polyacetal, also commonly referred to as polyoxymethylene, compositions are generally understood to include compositions based on homopolymers of formaldehyde, the terminal groups of which are end-capped by esterification of formaldehyde and other monomers that yield oxyalkylene groups with at least two adjacent carbon atoms in the main chain, the terminal groups of which copolymers can be hydroxyl terminate or can be end-capped by esterification or etherification. Compositions based on polyacetals of relatively high molecular weight, i.e., 10,000 to 100,000 are useful in preparing semi-finished and finished articles by any of the techniques commonly used with thermoplastic materials, e.g. compression molding, injection molding, extrusion, blow molding, rotational molding, melt spinning, stamping, thermoforming and the like. Finished articles made from such compositions possess desirable physical properties, including high stiffness, strength, low coefficient of friction, and good solvent resistance.
In some applications, polyacetal compositions are exposed to light for long periods of time. It is desired that said polyacetal compositions remain relatively stable upon exposure to light for such periods of time. To impart or improve the light stability of a polyacetal composition, a HALS may be added thereto. However, it is not uncommon that the addition of such a HALS can adversely affect other properties of the polyacetal composition, such as the melt processing thermal stability (e.g. gaseous formation evolution or discoloration during melt processing) of the composition and/or the ability of the composition to retain physical properties upon exposure to the heat and air. The present invention serves to alleviate such a problem.